Dr Whyte said he made the announcement with regret but the election result was clear and he must now turn to his career and family.

The ACT Party got only 0.69 percent of party votes in the election, although Mr Seymour won the Epsom seat following a deal with the National Party.

Mr Seymour said Dr Whyte rejuvinated ACT and under him membership increased, but it was not a surprise he stepped down.

"He faced quite a dilemma. He had given eight months of his time, full-time, for free, to campaign for the party. He has a family and a career to think about. He decided that, having not been elected to Parliament, he could not sustain that for another three years."

Mr Seymour said he understood the challenges that ACT faced.

"I refuse to believe that only 0.7 percent of New Zealanders want smaller government, more free enterprise and ultimately the more prosperous New Zealand that that brings," he said.

"Simultaneously, I accept that we are going to have to do much better at being a disciplined, collegial party that is positive about New Zealand and its future."

Jamie Whyte's full resignation statement

"Today I announce that I have tendered, and the board has accepted, my resignation as leader of ACT New Zealand.

"Clearly I make this announcement with regret, however the election result is clear, and I must now turn to my career and my family.

"I stood to lead ACT because I believe in the party's ideas. I will continue to advance these ideas both inside and outside the party. I do not rule out returning to a substantial role with ACT in the future."